33 research outputs found

### Partially Cooled Shocks: Detectable Precursors in the Warm/hot Intergalactic Medium

I present computations of the integrated column densities produced in the post-shock cooling layers and in the radiative precursors of partially cooled fast shocks as a function of the shock age. The results are applicable to the shock-heated warm/hot intergalactic medium which is expected to be a major baryonic reservoir and contain a large fraction of the so-called missing baryons. My computations indicate that readily observable amounts of intermediate and high ions, such as C IV, N V, and O VI, are created in the precursors of young shocks, for which the shocked gas remains hot and difficult to observe. I suggest that such precursors may provide a way to identify and estimate the "missing" baryonic mass associated with the shocks. The absorption-line signatures predicted here may be used to construct ion-ratio diagrams, which will serve as diagnostics for the photoionized gas in the precursors. In my numerical models, the time evolution of the shock structure, self-radiation, and associated metal-ion column densities are computed by a series of quasi-static models, each appropriate for a different shock age. The shock code used in this work calculates the non-equilibrium ionization and cooling, follows the radiative transfer of the shock self-radiation through the post-shock cooling layers, takes into account the resulting photoionization and heating rates, follows the dynamics of the cooling gas, and self-consistently computes the photoionization states in the precursor gas. I present a complete set of the age-dependent post-shock and precursor columns for all ionization states of the elements H, He, C, N, O, Ne, Mg, Si, S, and Fe as functions of the shock velocity, gas metallicity, and magnetic field. I present my numerical results in convenient online tables

### Metal-Ion Absorption in Conductively Evaporating Clouds

We present computations of the ionization structure and metal-absorption
properties of thermally conductive interface layers that surround evaporating
warm spherical clouds, embedded in a hot medium. We rely on the analytical
formalism of Dalton & Balbus to calculate the temperature profile in the
evaporating gas, and explicitly solve the time-dependent ionization equations
for H, He, C, N, O, Si, and S in the interface. We include photoionization by
an external field. We estimate how departures from equilibrium ionization
affect the resonance-line cooling efficiencies in the evaporating gas, and
determine the conditions for which radiative losses may be neglected in the
solution for the evaporation dynamics and temperature profile. Our results
indicate that non-equilibrium cooling significantly increases the value of the
saturation parameter at which radiative losses begin to affect the flow
dynamics. As applications we calculate the ion fractions and projected column
densities arising in the evaporating layers surrounding dwarf-galaxy-scale
objects that are also photoionized by metagalactic radiation. We compare our
results to the UV metal-absorption column densities observed in local
highly-ionized metal-absorbers, located in the Galactic corona or intergalactic
medium. Conductive interfaces significantly enhance the formation of high-ions
such as C^3+, N^4+, and O^5+ relative to purely photoionized clouds, especially
for clouds embedded in a high-pressure corona. However, the enhanced columns
are still too low to account for the O VI columns (~1e14 cm^-2) observed in the
local high-velocity absorbers. We find that O VI columns larger than ~1e13
cm^-2 cannot be produced in evaporating clouds. Our results do support the
conclusion of Savage & Lehner, that absorption due to evaporating O VI likely
occurs in the local interstellar medium, with characteristic columns of ~1e13
cm^-2.Comment: Accepted for Publication in Ap

### Ion-by-ion Cooling efficiencies

We present ion-by-ion cooling efficiencies for low-density gas. We use Cloudy
(ver. 08.00) to estimate the cooling efficiencies for each ion of the first 30
elements (H-Zn) individually. We present results for gas temperatures between
1e4 and 1e8K, assuming low densities and optically thin conditions. When
nonequilibrium ionization plays a significant role the ionization states
deviate from those that obtain in collisional ionization equilibrium (CIE), and
the local cooling efficiency at any given temperature depends on specific
non-equilibrium ion fractions. The results presented here allow for an
efficient estimate of the total cooling efficiency for any ionic composition.
We also list the elemental cooling efficiencies assuming CIE conditions. These
can be used to construct CIE cooling efficiencies for non-solar abundance
ratios, or to estimate the cooling due to elements not explicitly included in
any nonequilibrium computation. All the computational results are listed in
convenient online tables.Comment: Submitted to ApJS. Electronic data available at
http://wise-obs.tau.ac.il/~orlyg/ion_by_ion

### Radiative cooling in collisionally and photo ionized plasmas

We discuss recent improvements in the calculation of the radiative cooling in
both collisionally and photo ionized plasmas. We are extending the spectral
simulation code Cloudy so that as much as possible of the underlying atomic
data is taken from external databases, some created by others, some developed
by the Cloudy team. This paper focuses on recent changes in the treatment of
many stages of ionization of iron, and discusses its extensions to other
elements. The H-like and He-like ions are treated in the iso-electronic
approach described previously. Fe II is a special case treated with a large
model atom. Here we focus on Fe III through Fe XXIV, ions which are important
contributors to the radiative cooling of hot, 1e5 to 1e7 K, plasmas and for
X-ray spectroscopy. We use the Chianti atomic database to greatly expand the
number of transitions in the cooling function. Chianti only includes lines that
have atomic data computed by sophisticated methods. This limits the line list
to lower excitation, longer wavelength, transitions. We had previously included
lines from the Opacity Project database, which tends to include higher energy,
shorter wavelength, transitions. These were combined with various forms of the
g-bar approximation, a highly approximate method of estimating collision rates.
For several iron ions the two databases are almost entirely complementary. We
adopt a hybrid approach in which we use Chianti where possible, supplemented by
lines from the Opacity Project for shorter wavelength transitions. The total
cooling including the lightest thirty elements differs significantly from some
previous calculations

### Studies of multiple stellar systems - IV. The triple-lined spectroscopic system Gliese 644

We present a radial-velocity study of the triple-lined system Gliese 644 and
derive spectroscopic elements for the inner and outer orbits with periods of
2.9655 and 627 days. We also utilize old visual data, as well as modern speckle
and adaptive optics observations, to derive a new astrometric solution for the
outer orbit. These two orbits together allow us to derive masses for each of
the three components in the system: M_A = 0.410 +/- 0.028 (6.9%), M_Ba = 0.336
+/- 0.016 (4.7%), and $M_Bb = 0.304 +/- 0.014 (4.7%) M_solar. We suggest that
the relative inclination of the two orbits is very small. Our individual masses
and spectroscopic light ratios for the three M stars in the Gliese 644 system
provide three points for the mass-luminosity relation near the bottom of the
Main Sequence, where the relation is poorly determined. These three points
agree well with theoretical models for solar metallicity and an age of 5 Gyr.
Our radial velocities for Gliese 643 and vB 8, two common-proper-motion
companions of Gliese 644, support the interpretation that all five M stars are
moving together in a physically bound group. We discuss possible scenarios for
the formation and evolution of this configuration, such as the formation of all
five stars in a sequence of fragmentation events leading directly to the
hierarchical configuration now observed, versus formation in a small N cluster
with subsequent dynamical evolution into the present hierarchical
configuration.Comment: 17 pages, 9 figures, Accepted for publication in MNRA

### Equilibrium Configurations of Synchronous Binaries: Numerical Solutions and Application to Kuiper-Belt Binary 2001 QG298

We present numerical computations of the equilibrium configurations of
tidally-locked homogeneous binaries, rotating in circular orbits. Unlike the
classical Roche approximations, we self-consistently account for the tidal and
rotational deformations of both components, and relax the assumptions of
ellipsoidal configurations and Keplerian rotation. We find numerical solutions
for mass ratios q between 1e-3 and 1, starting at a small angular velocity for
which tidal and rotational deformations are small, and following a sequence of
increasing angular velocities. Each series terminates at an appropriate ``Roche
limit'', above which no equilibrium solution can be found. Even though the
Roche limit is crossed before the ``Roche lobe'' is filled, any further
increase in the angular velocity will result in mass-loss. For close,
comparable-mass binaries, we find that local deviations from ellipsoidal forms
may be as large as 10-20%, and departures from Keplerian rotation are
significant. We compute the light curves that arise from our equilibrium
configurations, assuming their distance is >>1 AU (e.g. in the Kuiper Belt). We
consider both backscatter (proportional to the projected area) and diffuse
(Lambert) reflections. Backscatter reflection always yields two minima of equal
depths. Diffuse reflection, which is sensitive to the surface curvature,
generally gives rise to unequal minima. We find detectable intensity
differences of up to 10% between our light curves and those arising from the
Roche approximations. Finally, we apply our models to Kuiper Belt binary 2001
QG298, and find a nearly edge-on binary with a mass ratio q = 0.93
^{+0.07}_{-0.03}, angular velocity Omega^2/G rho = 0.333+/-0.001 (statistical
errors only), and pure diffuse reflection. For the observed period of 2001
QG298, these parameters imply a bulk density, rho = 0.72 +/- 0.04 g cm^-3.Comment: Accepted to Ap